1. Field of the Invention
The invention relates to new and improved symmetrical component filters for deriving signals responsive to sequence quantities of a three-phase electrical power system.
2. Description of the Prior Art
The method of symmetrical components is a valuable engineering tool for the relay engineer, and is directly responsible for the conception of many protective relays and circuits in common usage today. The use of positive sequence alone is not very selective between faults and normal loads and swings. The use of negative sequence alone would not protect against three-phase faults. The use of zero sequence alone would not protect for all phase faults. Thus, while certain types of protective relays require only a single sequence quantity, others utilize two or all three of the symmetrical components to provide improved sensitivity and operation for all types of faults.
In order for the protective relays based on symmetrical components to operate, it must receive signals responsive to the required sequence, or sequences. Sequence filters provide these signals. Since the zero sequence quantities are all in phase with each other, the zero sequence filter does not require timing components. Thus, the zero sequence filters, for either current or voltage, are easily constructed of current or voltage transformers. In practice, the zero sequence signals are obtained from the current and/or voltage transformers already required for obtaining phase-related signals from the electrical power system for other purposes. Thus, signals responsive to the zero sequence are obtained for little or no additional cost.
Positive and negative sequence filters, on the other hand, have a 120 degree phase shift with respect to each other, and this time significance adds greatly to their complexity and cost. Positive and negative sequence filters are usually constructed of reactors, auto transformers, transformers, resistors, or capacitors, and they are thus bulky and costly. Active sequence filter networks using operational amplifiers have been developed in an attempt to reduce the size and cost of the positive and negative sequence filters. The translation of the functions performed by passive elements into active sequence filters, however, has resulted in many disadvantages. For example, certain of the prior art filters require interrelated adjustments to be performed, requiring tedious calibration since changing one setting affects the other. Odd ratios between the input signals are often required to be developed in order to achieve the phase shifts required by the specific implementation of the filter. Positive and negative sequence current filters of the active type may also reproduce the zero sequence in the output signal, unless the zero sequence current is removed from the signals before they are applied to the active filter; or, the active filter must contain special circuitry in order to cancel the zero sequence component.
Further, when discrete or separate positive and negative sequence signals are both required, a completely separate filter is used for each signal. This is true for both the passive and active sequence filters.
It would be desirable to reduce the cost of positive and negative symmetrical component sequence filters, without incurring the offsetting disadvantages of difficult calibration. It would also be desirable to be able to provide a sequence current filter which rejects any zero sequence component in the input signal without requiring additional circuitry specifically to deal with the zero sequence component. It would also be desirable to reduce the cost of positive and negative symmetrical component sequence filters when both are required by utilizing certain of the processing circuitry in common. It would also be desirable to reduce the cost of positive and negative sequence filters by reducing the number of reactive elements and the number of phase shifts utilized, other than 180 phase shifts which are easily performable by transformers or operational amplifiers. Finally, it would be desirable to reduce the cost of positive and negative sequence filters by eliminating the requirement of developing odd decimal relationships between the phase-related input signals.